net.sf.saxon.expr
Class Optimizer

java.lang.Object
  net.sf.saxon.expr.Optimizer

All Implemented Interfaces:

Serializable

Direct Known Subclasses:

OptimizerSA

public class Optimizer
extends Object
implements Serializable

This class performs optimizations that vary between different versions of the Saxon product. The optimizer is obtained from the Saxon Configuration. This class is the version used in Saxon-B, which in most cases does no optimization at all: the methods are provided so that they can be overridden in Saxon-SA.

See Also:

Serialized Form

Constructor Summary

Optimizer(Configuration config)
Create an Optimizer.

Method Summary

Expression	convertPathExpressionToKey(PathExpression pathExp, ExpressionVisitor visitor) Examine a path expression to see whether it can be replaced by a call on the key() function; if so, generate an appropriate key definition and return the call on key().
FilterExpression	convertToFilterExpression(PathExpression pathExp, TypeHierarchy th) Convert a path expression such as a/b/c[predicate] into a filter expression of the form (a/b/c)[predicate].
Expression	extractGlobalVariables(Expression body, ExpressionVisitor visitor) Extract subexpressions from the body of a function that can be evaluated as global variables
Configuration	getConfiguration() Get the Saxon configuration object
int	isIndexableFilter(Expression filter) Test whether a filter predicate is indexable.
Value	makeClosure(Expression expression, int ref, XPathContext context) Make a Closure, given the expected reference count
Expression	makeConditionalDocumentSorter(DocumentSorter sorter, PathExpression path) Make a conditional document sorter.
BinaryExpression	makeGeneralComparison(Expression p0, int op, Expression p1, boolean backwardsCompatible) Create a GeneralComparison expression
ValueRepresentation	makeIndexedValue(SequenceIterator iter) Create an indexed value
ValueRepresentation	makeSequenceExtent(Expression expression, int ref, XPathContext context) Make a SequenceExtent, given the expected reference count
Expression	optimizeCopy(Expression select) Attempt to optimize a copy operation.
Expression	promoteExpressionsToGlobal(Expression body, ExpressionVisitor visitor) Identify expressions within a function or template body that can be promoted to be evaluated as global variables.
void	trace(String message) Trace optimization actions
void	trace(String message, Expression exp) Trace optimization actions
Expression	tryIndexedFilter(FilterExpression f, ExpressionVisitor visitor, boolean indexFirstOperand) Try converting a filter expression to a call on the key function.
Expression	tryInlineFunctionCall(UserFunctionCall functionCall, ExpressionVisitor visitor, ItemType contextItemType) Replace a function call by the body of the function, assuming all conditions for inlining the function are satisfied
Expression	trySwitch(Choose choose, StaticContext env) Try to convert a Choose expression into a switch

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Optimizer

public Optimizer(Configuration config)

Create an Optimizer.

Parameters:

config - the Saxon configuration

Method Detail

getConfiguration

public Configuration getConfiguration()

Get the Saxon configuration object

Returns:

the configuration

makeGeneralComparison

public BinaryExpression makeGeneralComparison(Expression p0,
                                              int op,
                                              Expression p1,
                                              boolean backwardsCompatible)

Create a GeneralComparison expression

Parameters:

p0 - the first operand

op - the operator

p1 - the second operand

backwardsCompatible - true if XPath 1.0 backwards compatibility is in force

Returns:

the constructed expression

optimizeCopy

public Expression optimizeCopy(Expression select)
                        throws XPathException

Attempt to optimize a copy operation. Return null if no optimization is possible.

Parameters:

select - the expression that selects the items to be copied

Returns:

null if no optimization is possible, or an expression that does an optimized copy of these items otherwise

Throws:

XPathException

makeClosure

public Value makeClosure(Expression expression,
                         int ref,
                         XPathContext context)
                  throws XPathException

Make a Closure, given the expected reference count

Parameters:

expression - the expression to be evaluated

ref - the (nominal) number of times the value of the expression is required

context - the XPath dynamic evaluation context

Returns:

the constructed Closure

Throws:

XPathException

makeSequenceExtent

public ValueRepresentation makeSequenceExtent(Expression expression,
                                              int ref,
                                              XPathContext context)
                                       throws XPathException

Make a SequenceExtent, given the expected reference count

Parameters:

expression - the expression to be evaluated

ref - the (nominal) number of times the value of the expression is required

context - the XPath dynamic evaluation context

Returns:

the constructed Closure

Throws:

XPathException

convertPathExpressionToKey

public Expression convertPathExpressionToKey(PathExpression pathExp,
                                             ExpressionVisitor visitor)
                                      throws XPathException

Examine a path expression to see whether it can be replaced by a call on the key() function; if so, generate an appropriate key definition and return the call on key(). If not, return null.

Parameters:

pathExp - The path expression to be converted.

visitor - The expression visitor

Returns:

the optimized expression, or null if no optimization is possible

Throws:

XPathException

tryIndexedFilter

public Expression tryIndexedFilter(FilterExpression f,
                                   ExpressionVisitor visitor,
                                   boolean indexFirstOperand)

Try converting a filter expression to a call on the key function. Return the supplied expression unchanged if not possible

Parameters:

f - the filter expression to be converted

visitor - the expression visitor, which must be currently visiting the filter expression f

indexFirstOperand - true if the first operand of the filter comparison is to be indexed; false if it is the second operand

Returns:

the optimized expression, or the unchanged expression f if no optimization is possible

convertToFilterExpression

public FilterExpression convertToFilterExpression(PathExpression pathExp,
                                                  TypeHierarchy th)
                                           throws XPathException

Convert a path expression such as a/b/c[predicate] into a filter expression of the form (a/b/c)[predicate]. This is possible whenever the predicate is non-positional. The conversion is useful in the case where the path expression appears inside a loop, where the predicate depends on the loop variable but a/b/c does not.

Parameters:

pathExp - the path expression to be converted

th - the type hierarchy cache

Returns:

the resulting filterexpression if conversion is possible, or null if not

Throws:

XPathException

isIndexableFilter

public int isIndexableFilter(Expression filter)

Test whether a filter predicate is indexable.

Parameters:

filter - the predicate expression

Returns:

0 if not indexable; +1 if the predicate is in the form expression=value; -1 if it is in the form value=expression

makeIndexedValue

public ValueRepresentation makeIndexedValue(SequenceIterator iter)
                                     throws XPathException

Create an indexed value

Parameters:

iter - the iterator that delivers the sequence of values to be indexed

Returns:

the indexed value

Throws:

UnsupportedOperationException: - this method should not be called in Saxon-B

XPathException

makeConditionalDocumentSorter

public Expression makeConditionalDocumentSorter(DocumentSorter sorter,
                                                PathExpression path)

Make a conditional document sorter. This optimization is attempted when a DocumentSorter is wrapped around a path expression

Parameters:

sorter - the document sorter

path - the path expression

Returns:

the original sorter unchanged when no optimization is possible, which is always the case in Saxon-B

tryInlineFunctionCall

public Expression tryInlineFunctionCall(UserFunctionCall functionCall,
                                        ExpressionVisitor visitor,
                                        ItemType contextItemType)

Replace a function call by the body of the function, assuming all conditions for inlining the function are satisfied

Parameters:

functionCall - the functionCall expression

visitor - the expression visitor

contextItemType - the context item type

Returns:

either the original expression unchanged, or an expression that consists of the inlined function body, with all function parameters bound as required. In Saxon-B, function inlining is not supported, so the original functionCall is always returned unchanged

promoteExpressionsToGlobal

public Expression promoteExpressionsToGlobal(Expression body,
                                             ExpressionVisitor visitor)
                                      throws XPathException

Identify expressions within a function or template body that can be promoted to be evaluated as global variables.

Parameters:

body - the body of the template or function

visitor - the expression visitor

Returns:

the expression after subexpressions have been promoted to global variables

Throws:

XPathException

trySwitch

public Expression trySwitch(Choose choose,
                            StaticContext env)

Try to convert a Choose expression into a switch

Parameters:

choose - the Choose expression

env - the static context

Returns:

the result of optimizing this (the original expression if no optimization was possible)

extractGlobalVariables

public Expression extractGlobalVariables(Expression body,
                                         ExpressionVisitor visitor)
                                  throws XPathException

Extract subexpressions from the body of a function that can be evaluated as global variables

Parameters:

body - the body of the function

Returns:

a reference to the new global variable if a variable has been created, or null if not

Throws:

XPathException

trace

public void trace(String message,
                  Expression exp)

Trace optimization actions

Parameters:

message - the message to be displayed

exp - the expression after being rewritten

trace

public void trace(String message)

Trace optimization actions

Parameters:

message - the message to be displayed